An inkjet printing method is to eject inks onto a printing medium for printing from a printing portion including a printing head. When compared with other printing methods, the inkjet printing method has various advantages such as easiness of high definition and high speed printing as well as excellent quietness and inexpensiveness. In recent years, with an increase in the demand of color printing, various types of color inkjet printing apparatuses capable of printing in high image quality comparable to that of the silver halide photography have been developed.
In an inkjet printing apparatus to which such a inkjet printing method is applied, high definition and high speed printing is achieved using a printing head including an ejection portion in which a large number of nozzles are integratedly arranged at a high density. Further, in a color inkjet printing apparatus, a printing head having multiple ejection portions is usually used in order to eject inks with different color tones (colors and densities). A nozzle is herein a term for a combination of an ejection opening through which an ink is ejected; a liquid passage communicating with the ejection openings and an energy generation element which is disposed in this passage or the like and generates energy to be used for the ejection of ink.
For inkjet printing apparatuses, there are a so-called line printer type and a so-called serial printer type. For those widespread in general, the latter one has come to the mainstream. In the latter one, an image is formed by alternately performing main scanning and sub-scanning. In the main scanning, the printing head ejects inks while moving over a printing medium in a direction different from that of the arrangement of ejection openings. In the sub-scanning, the printing medium is moved, relative to the printing head, in a direction perpendicular to that of the main scanning. To achieve further high speed printing, such a serial printer type inkjet printing apparatus is designed to perform bidirectional printing in which a printing operation is performed in both forward main scanning and backward main scanning.
FIG. 1 is a schematic front view showing an example of a configuration of an inkjet printing head (hereinafter, also simply referred to as a printing head) 101 which is used in an inkjet printing apparatus of the serial printer type. This printing head 101 includes multiple ejection portions 102 enabling the ejection of inks with different color tones. In the example shown in the drawing, four ejection portions 102 are provided corresponding to, for example, four colors of inks, i.e., inks of black (K), cyan (C), magenta (M), and yellow (Y).
In each ejection portion 102, nozzles 103 each including an ejection opening 103A and a liquid passage 103B are disposed at predetermined intervals on both sides of an ink supplying passage 105 opened in a substrate 100. The nozzle arrays are disposed to have a relationship in which the nozzles in one of the nozzle arrays are shifted from the nozzles in the other nozzle array by half a pitch, thereby achieving resolution twice as high as that achieved by each nozzle array alone. To the liquid passage 103B, a heater (not shown) which generates thermal energy in response to, for example, electrification is provided as an energy generation element. This heater rapidly heats an ink and thereby evaporates the ink. With pressure due to a bubble thus generated, the ink is ejected through the ejection opening 103A, for example, as a droplet. The generated bubble is cooled with the surrounding ink and, thereby, vapor of the ink inside the bubble is condensed back into liquid. Thus, the bubble eventually disappears. At this time, an amount of ink corresponding to that consumed for the ejection is refilled into the liquid passage 103B through the ink supplying passage 105.
FIG. 2 schematically shows a configuration of a main part of the inkjet printing apparatus using the foregoing printing head. In this figure, reference numerals 201 denote ink tanks of cartridge type respectively containing the above-described four colors of ink, and the ink tanks 201 are independently and detachably attachable to the printing head 101 including the foregoing four ejection portions 102.
Reference numeral 206 denotes a carriage which performs the reciprocating movement (this movement is referred to as a main scanning or a scan, and a direction of the reciprocating movement is also referred to as a main scanning direction) in an X-direction and its opposite direction of the drawing of FIG. 2 while supporting the printing head 101 and the four ink tanks 201. Reference numeral 203 denotes a conveying roller. This conveying roller 203 rotates in the direction of an arrow of FIG. 2, while nipping a printing medium P with the auxiliary roller 204, so that the printing medium P is intermittently conveyed (sub-scanning) in the Y direction at intervals between the main scanning operations. Reference numerals 205 denote a pair of paper-feeding rollers for feeding the printing medium. As in the case of rollers 203 and 204, the pair of rollers 205 rotate while nipping the printing medium P therebetween. At this time, a tensile force is created in the printing medium by setting a rotating speed of the rollers 205 to be lower than that of the conveying roller 203, so that the printing medium can be conveyed without slack.
The carriage 206 waits at a home position h shown in a dashed line of FIG. 2 when a printing operation by the printing head 101 is not performed, or when a recovery operation for the printing head 101 is performed.
Further, once a print start instruction is given, the carriage 206 staying at the home position h before a start of printing is scanned in a forward direction (X-direction), while causing the printing head 101 to perform an ejection operation from the nozzles, thereby printing a certain swath corresponding to a nozzle arrangement range. Once the main scanning up to an end of the printing medium is completed, the printing medium P is conveyed by an amount equivalent to the swath, and at the same time, the carriage 206 is returned to the home position h to perform again the main scanning in the X-direction for making printing. In this manner, the printing for one swath by using the printing head 101 in a single main scanning of the carriage 206, and the conveying of a printing medium by an amount equivalent to one swath after the single main scanning are repeatedly performed, so that, for example, one page printing can be completed. In this case, one printing region on the printing medium P is completed with one-time main scanning, and such a printing mode is referred to as a one-pass printing mode.
In contrast, in some cases, instead of performing the conveying by an amount equivalent to a swath for each single main scanning, the conveying is performed after the main scanning is performed multiple times. Alternatively, in other some cases, an image is completed by performing multiple times of main scanning and conveying and by involving different nozzles with printing for a single image region. More specifically, the multiple times of main scanning and conveying include: performing printing based on data thinned out by a predetermined mask in each main scanning; performing the conveyance of printing medium by an amount equivalent to 1/n swath; and then performing the next main scanning again. The printing mode described above is referred to as multipass printing. That is, this multipass printing mode is a printing mode in which inks are applied to a single image region in multiple times to complete an image, and it is known in general that the larger the number of passes, the better the printing quality.
Unidirectional printing has been described above in which a printing operation is performed only at the time of moving the carriage 206 in the forward direction; however, bidirectional printing can also be performed in which a printing operation is performed also during the backward direction movement when high-speed printing is performed.
In a case of adopting an inkjet printing method using thermal energy to perform an ink ejection, a uniform and continuous drive of a heater causes rise in temperatures of a printing head and ink. It is known that such temperature rise lowers the viscosity of the ink, and thereby causes a larger amount of ink to be ejected even by the driving under the same condition, which creates density unevenness.
The reduction of density unevenness is important to perform high quality printing. As typical means for achieving this reduction, there is means for controlling an ejection amount so that the ejection amount may be constant, or means for correcting data for printing, themselves. Further, as means for reducing density unevenness, there are known a technique using the above mentioned multipass printing, or a technique by reducing a drive frequency of the heater or the speed of the main scanning although such reduction causes a decrease in recoding speed.
Here, when the same drive pulses are applied to a heater, an ejection amount from a printing head depends on the temperature of an ink in the vicinity of the heater. Therefore, the management of temperature of the ink is strongly desired, but this is difficult in practice. For this reason, a currently-widespread technique for controlling an ejection amount from a printing head is targeted to control the temperature of the printing head instead of the ink temperature.
For example, Patent Document 1 discloses a technique in which a sensor for the detection of the temperature inside a printing head is disposed in the printing head, and an output of this sensor is monitored to modulate a drive pulse. More specifically, a control method (PWM control) has been proposed in which once the temperature rises, a period of drive time (heating time) of a heater is reduced by changing a pulse width of a pulse signal for driving the heater or by performing a similar operation, whereby a rise in temperature of the head is restrained to cause an ejection amount of ink to be constant.
However, the sensor is attached in the vicinity of the head, and a precise output is not capable of being monitored with an MPU (CPU) due to a noise caused by a drive of the printing head, so that there have been problems that a precise temperature control is not capable of being made, and the controlling of an ejection amount is not sufficient. In this connection, besides the configuration in which a temperature sensor is provided to a printing head, use of a technique has been proposed which includes an amplification mechanism of a detected temperature output, a measure against noise for a detection result, and the like. However, this increases cost by that much. Accordingly, in light of the reliability of a sensor, a technique has been proposed in which the temperature of the printing head is estimated on the basis of image data to be printed, and it has also been proposed that this technique is substituted for or is used along with the technique for detecting temperature. For example, prior to a main scanning, image data for a single main scanning are temporarily stored in a memory area such as an image buffer; the number of valid data in the image buffer are counted; and a change in the temperature of the head is estimated using the count result. Then, the modulation of a pulse width of a drive signal, or the like is performed to thereby perform a main scanning.
Further, as disclosed in Patent Documents 2 and 3, there is a technique in which a temperature is acquired using means for acquiring a temperature around a printing apparatus or a printing head by using a sensor or the like, and means for estimating a temperature rise of an inkjet head on the basis of an amount of heat inputted into the printing head per unit time.
Further, in recent years, it has been strongly desired that a high-precision technique be used over a conventional estimation method, because of an increase in an ejection frequency with the increase of a printing speed, and of an increase in the number of nozzles per nozzle array. High precision temperature estimation is achieved by shortening time intervals for a temperature estimation calculation, but the shortening of the time intervals increases a calculation load on a printing apparatus. Thus, it becomes necessary to improve the capability of an MPU (CPU) being a calculation unit, or decrease in throughput occurs.
For these problems, Patent Document 3 discloses, as a temperature estimation method having a less calculation load with a high accuracy, a technique in which the temperature of a printing head is estimated on the basis of a drive condition of the printing head, and depending on this estimated temperature, the foregoing PWM control is performed, so that a precise control on an ejection amount is performed. More concretely, the drive condition of the printing head is converted into an amount of input heat to be stored in the printing head, and the storage of heat after the radiation of heat due to the elapse of unit time is calculated using heat in the printing head stored up to the last main scanning. Thereafter, the storage of heat of the printing head is stored for each thermal time constant, and each amount of input heat and an amount of heat after heat radiation are added, so that a temperature rise of the printing head is calculated.
On the other hand, Patent Document 4 discloses a technique in which, in a printing apparatus performing a print on a large-sized printing medium, a temperature estimation and an ejection amount control are performed in real-time using image data. More concretely, disclosed are: a technique in which valid data in image data are counted, and when the count value attains a predetermined value, the width of a pulse signal for driving the head is changed, or print data are thinned out by a predetermined amount to correct data and the corrected data are printed.
In recent years, with the spread of personal computers and digital cameras, it has been strongly requested that further high definition and further speeding-up on printing is performed in printing apparatuses serving as image output terminals. For the inkjet printing apparatuses, to cope with the request on the high definition printing, one has come out in which, to make printing, dots with small diameters are formed using a printing head densely equipped with finer nozzles through which smaller amounts of ink are ejected. In performing printing using such an inkjet printing apparatus, the number of ink dots with which a printing region is covered has a large influence on the size of the printing region.
This will be explained using FIGS. 3A and 3B. Let us suppose that there are a printing head forming a dot shown in FIG. 3A, and another printing head simply with a half the dot diameter in FIG. 3A, as shown in FIG. 3B. In this case, in order to perform printing on the same printing area, the number of dots disposed in each of the longitudinal and lateral directions is twice as many as that of dots of FIG. 3A, and thus, the total number of dots disposed in FIG. 3B is four times as many as that of the case of FIG. 3A. Therefore, when a printing head forming dots such as those in FIG. 3B is driven under the same condition as that for another printing head forming dots such as those in FIG. 3A, it is natural that the printing speed is extremely low.
To avoid the reduction of the printing speed, applied are: a method in which an ink ejection frequency (a drive frequency of a heater) and the speed of a main scanning are increased, and a method in which the number of passes is reduced in performing multipass printing.
However, an increase in the drive frequency of the heater causes a rise of the temperature of the printing head to be marked, resulting in density unevenness due to an increase of the ejection amount. Further, with dots having a small diameter, the number of dots printed on a printing region also increases, so that the density unevenness is further visible due to increase in the ejection amount of each nozzle. Still further, when the next main scanning is performed with the temperature of the head remaining high due to the previous one-time main scanning, the ejection amount increases to a level higher than the level of any previous ejection amount, and density unevenness occurs for each main scanning. In addition, even if the number of passes is reduced, the number of ink dots formed in one-time main scanning increases and therefore, in such a case also, an increase of the ejection amount due to an increase of the temperature of the printing head causes density unevenness on a printing region. Further, when the drive frequency improvement and the pass number reduction are performed at the same time, it is natural that the influence thereof becomes considerably large.
Under such circumstances, a control which involves the detecting of the temperature of the printing head using a temperature sensor as conventional has a problem in responsiveness. In addition, the increase in the ejection frequency results in reduction in maximum pulse width in one ejection timing. Therefore, when the detection of the temperature of the printing head is performed, or when the temperature estimation is performed on the basis of data to be printed, a controllable range of the ejection amount in a modulatable range of a pulse width is narrowed, so that controlling capability becomes insufficient.
Furthermore, especially, in a case of performing high speed printing, such as the case where bidirectional printing is applied in a one-pass printing mode, density unevenness may occur even within a region for a single main scanning.
This will be described with reference to FIG. 4. For example, consider the case where bidirectional printing is performed in the foregoing one-pass printing mode. In such a case, a density distribution comes up in the main scanning direction on a region on which printing is performed for each main scanning. To be more specific, a band-like density unevenness occurs for each main scanning, and especially, the density increases from a start portion of each main scanning toward an end portion thereof.
FIGS. 5A, 5B, and 5C are schematic views respectively showing a state of a printing region on which a “solid” image having the same gradation has been printed in an arbitrary main scanning in one-pass printing mode, the temperature of a printing head at that time, and an ejection amount at that time. With the progress of printing in the main scanning direction by the printing head, the temperature Th of the printing head increases as shown in FIG. 5B, and with this increase in the temperature, the ejection amount Vd also increases as shown in FIG. 5C. As a result, as shown in FIG. 5A, density unevenness occurs in a direction along the main scanning direction.
None of the conventional techniques disclosed in Patent Documents 1 to 4 enables the controlling of effectively suppressing such density unevenness.    Patent Document 1: Japanese Patent Laid-Open No. H 5-31905 (1993)    Patent Document 2: Japanese Patent Laid-Open No. H 5-208505 (1993)    Patent Document 3: Japanese Patent Laid-Open No. H 7-125216 (1995)    Patent Document 4: Japanese Patent Laid-Open No. H 8-156258 (1996)